The coupling of light between core and cladding layers of optical waveguides influences spectral transmission characteristics of the waveguides. External controls modify coupling parameters to change or xe2x80x9ctunexe2x80x9d the transmission characteristics.
Spectral transmission characteristics are influenced by waveguide (e.g., fiber) materials and geometries; in-line optical devices including routers, filters, and amplifiers: and environmental factors, as well as various interactions with the signals themselves. Optical amplifiers often produce substantially unequal gain over a operating spectral range and are paired with gain flattening filters including thin films, long period gratings, and fiber Bragg gratings to preserve desired spectral characteristics.
Additional gain flattening is sometimes needed at a system level, particularly in undersea applications. Gain ripples of amplifiers chained together as well as other in-line system anomalies accumulate and produce a system ripple that can also be mitigated by other gain flattening filters. The filters have fixed spectral responses that can be matched to particular system requirements. However, some system-wide changes, as well as other changes more pronounced locally, vary over time. Changing environmental conditions and system perturbations, including aging of the system, can alter the spectral transmission profiles of systems in transient and unpredictable ways. Dynamic tuning of the spectrum is needed to maintain system stability.
Tunable filters, particularly tunable fiber Bragg gratings, are available with spectral responses that can be shifted along the spectrum. Filter gratings are tuned by varying their periodicity under the control of an external force such as compression or stress. However, the system spectral transmission characteristics that vary over time are not easily counteracted by the shifting of narrow attenuation bands. Especially with respect to closely spaced signals along the spectrum, shifting attenuation bands can disturb adjacent signals. Some short period gratings attenuate spectral bands by reflection, which require additional system complexity to accommodate or remove the reflected light.
Improved tuning capabilities are provided in accordance with our invention largely by scaling filter responses rather than shifting the responses in wavelength. Within predetermined wavelength domains, the overall amplitudes of attenuated spectral bands can be increased or decreased without significantly altering the central wavelengths of the attenuated bands. The bands can be attenuated by dissipation rather than reflection, and our new amplitude tunable filters can be combined with other tunable or passive filters to provide a broader system response or to individually adjust transmission characteristics of different channels.
Our preferred embodiments rely on a special coupling mechanism having advantages for dynamic tuning that have been overlooked until now. Long period gratings, tapered pathways, and other perturbations can be used along optical waveguides to couple light from cores into surrounding claddings. Ambient conditions surrounding the claddings affect the cladding modes.
For example, a paper entitled xe2x80x9cDisplacements of the resonant peaks of a long-period fiber grating induced by a change of ambient refractive indexxe2x80x9d published in Optics Letters, Vol. 22, No. 23, on Dec. 1, 1997, discusses the effects of surrounding the cladding of a long period grating with mediums having different ambient refractive indices. Below the refractive index of the cladding, the cladding mode attenuation bands shift toward the lower wavelengths and decrease in amplitude as the ambient index approaches the cladding index. The attenuation bands substantially disappear at an ambient index matching the cladding index because the cladding modes are no longer guided. Above the refractive index of the cladding, the attenuation bands reappear and increase in amplitude with further increases in the ambient index but do not shift in wavelength. We exploit this latter spectral coupling behavior in the preferred embodiments of our new amplitude tunable filter.
One such embodiment of our amplitude tunable filter has a waveguide including a core and cladding that guide a light beam having a range of wavelengths along the core. A coupler couples at least one band of the wavelengths from the core into the cladding. An overcladding covers at least a portion of the cladding and exhibits a refractive index that is higher than a refractive index of the cladding. In addition, the refractive index of the overcladding is subject to change by an external control. A controller for this purpose adjusts the refractive index of the overcladding within a range that is higher than the refractive index of the cladding to vary the amplitude of the cladding-coupled band without significantly shifting the central wavelength of the band.
The coupler is preferably a long period grating formed along the waveguide but could also be a tapered or lattice filter, a fused fiber device, or other coupling structure that couples core transmissions into cladding modes. The coupler is located along the waveguide within a region covered by the overcladding and is preferably athermalized to inhibit a shift in the central wavelength of the coupled band as a function of waveguide temperature. In contrast, the overcladding preferably exhibits a refractive index that varies substantially as a function of temperature. In a preferred embodiment the overcladding is an organic containing optical material having a negative dn/dT. A particularly preferred embodiment of the overcladding is an inorganic-organic hybrid material, referred to as a hybrid sol gel material, having a refractive index (e.g., 1.47-1.55) that is substantially higher than a typical silica cladding and a rate of index change with temperature (dn/dT) of approximately xe2x88x923xc3x97104. The hybrid material preferably includes an extended matrix containing silicon and oxygen atoms with at least some of the silicon atoms being directly bonded to substituted or unsubstituted hydrocarbon moieties. In a solid form, the hybrid material provides structural support and protection for the underlying layers of core and cladding and can also be formulated to resist bending for safeguarding grating performance.
The controller preferably operates a temperature conditioner that adjusts the temperature of the overcladding to vary the refractive index of the overcladding within the range that is higher than the refractive index of the cladding. For example, the temperature conditioner can be formed as a resistive heater in thermal contact with the overcladding. Other heaters or coolers could be used to produce a desired range of temperature variation in the overcladding corresponding to a range of overcladding refractive indices that are above the cladding.
Our invention can also be described as a system that adjusts amplitudes of cladding mode coupled wavelengths of a light beam without significantly shifting the coupled wavelengths of the beam. The adjustment is accomplished by modifying a coupling between a core and a cladding with an overcladdihg having a refractive index that is higher than a refractive index of the cladding. The overcladding refractive index is subject to change as a function of an external control that is provided for varying the refractive index of the overcladding within a range of refractive indices that are greater than the refractive index of the cladding.
A baseline system-wide spectral response is preferably provided by a combination of passive filters. Variations of the system response from the baseline are counteracted by one or more amplitude tunable filters. Preferably, a plurality of amplitude tunable filters are concatenated in line with the passive filters to produce a combined spectral response. Each of the amplitude tunable filters preferably attenuates a substantially unique band of the system spectrum. Amplitudes of the attenuated bands can be individually adjusted to maintain desired spectral transmission characteristics.